Silicon carbide (SiC) consisting of the elements belonging to the 14th group (IVb group) is known as one of the IV-IV group compound semiconductor materials. Since it is possible to obtain a silicon oxide (SiO2) film from silicon carbide, voltage-driven-type devices having a MOS structure constituted of a metal, an oxide film, and a semiconductor are realized. The realization of the voltage-driven-type devices having a MOS structure by employing silicon carbide is very advantageous for the device application technologies.
As compared with silicon, silicon carbide is characterized in the following aspects. The band gap of silicon carbide is wider than that of silicon. The saturation drift velocity of silicon carbide is faster than that of silicon. The dielectric breakdown voltage of silicon carbide is higher than that of silicon. The thermal conductivity of silicon carbide is higher than that of silicon. Therefore, the device that employs silicon carbide (hereinafter referred to as the “silicon carbide device”) is more advantageous than the device that employs silicon (hereinafter referred to as the “silicon device”), since the silicon carbide device facilitates reducing the electric power consumption, since the silicon carbide device can be used in a higher-temperature environment, and since the silicon carbide device facilitates operating faster.
Therefore, the silicon carbide device facilitates improving the performances thereof more effectively than the silicon device. Therefore, silicon carbide is expected to be a semiconductor material of the next generation beyond the limit of silicon. As the silicon carbide devices, Schottky barrier diodes, MOSFETs and such devices have been developed and put into market.
In the voltage-driven-type devices having a MOS structure and employing silicon carbide, a silicon oxide film, that will work as a gate insulator film, for example, is formed on a substrate made of silicon carbide (hereinafter referred to as a “silicon carbide substrate” or sometimes as a “SiC substrate”). Therefore, it is necessary to improve the reliability of the silicon oxide film and the interface properties at the interface between the silicon oxide film and the silicon carbide substrate to be high enough to make the voltage-driven-type devices having a MOS structure to be used practically.
A method of forming a silicon oxide film on a 4H—SiC substrate by dry oxidation has been proposed (see, for example, N. S. Saks, S. S. Mani, and A. K. Agarwal, “Interface trap profile near the band edges at the 4H—SiC/SiO2 interface”, APPLIED PHYSICS LETTERS, 17 Apr. 2000, VOL. 76, NUMBER 16, pp. 2250-2252).
Another method that preliminarily treats the surface of a silicon carbide substrate using an ammonia (NH3) gas and deposits a silicon oxynitride (SiON) film by the chemical vapor deposition (CVD) method on the silicon carbide substrate treated preliminarily has been proposed (see, for example, Y. Iwasaki, H. Yano, T. Hatayama, Y. Uraoka, and T. Fuyuki, “NH3 Plasma Pretreatment of 4H—SiC(000-1) Surface for Reduction of Interface States in Metal-Oxide-Semi conductor Devices”, Applied Physics Express 3, Japan Society of Applied Physics, 2010, 026201, pp. 1-3).
Still another method is described by M. Noborio, J. Suda, and T. Kimoto, “4H—SiC MIS Capacitors and MISFETs With Deposited SiNx/SiO2 Stack-Gate Structures”. IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 55, NO. 8. AUGUST 2008, pp. 2054-2060. First, a silicon nitride (Si3N4) film is grown by the CVD method on a silicon carbide substrate. Then, a silicon oxide film is deposited on the silicon nitride film. Then, annealing is conducted in a nitrogen gas (N2) atmosphere at 1300° C.
A method of manufacturing a semiconductor device that includes a transition layer, 1 nm or less in thickness, at the interface between a SiC substrate and a gate insulator film is proposed in Japanese Patent No. JP 4647211. The method proposed in Japanese Patent No. JP 4647211 includes the step of oxidizing a part of the SiC substrate for forming a first insulator film; the step of forming a second insulator film above the first insulator film; the step of forming a gate electrode above a part of the SiC substrate with the first and second insulator films sandwiched between the SiC substrate and the gate electrode, and the step of forming a source region and a drain region in a part of the SiC substrate.
The method further includes, in subsequent to the step for forming the first insulator film, the step of annealing the wafer in a nitrogen monoxide atmosphere under the pressure of 1.4×10 Pa or higher and the atmospheric pressure or lower, at a temperature between 600° C. and 1400° C., and for 1 hr or longer and 3 hr or shorter. In the step of forming the second insulator film, the second insulator film is formed by the CVD method at a temperature between 600° C. and 1000° C. but lower than the above-described annealing temperature in an atmosphere containing silane and dinitrogen monoxide. After the step of forming the second insulator film, the annealing is conducted at a temperature between 600° C. and 1000° C. but lower than the temperature, at which the second insulator film is formed, in an atmosphere containing nitrogen monoxide.
Although various methods have been proposed for forming a silicon oxide film on a silicon carbide substrate, any method that fully brings out the intrinsic properties of silicon carbide has not been established yet. For example, the conventional methods described above can not obtain, at the interface between a silicon oxide film and a silicon carbide substrate, any interface property excellent enough to make voltage-driven-type devices having a MOS structure to be used practically.
The interface properties at the interface between a silicon oxide film and a silicon carbide substrate affect adversely the channel mobility and the threshold voltage Vth of the voltage-driven-type devices having a MOS structure. For obtaining interface properties excellent enough to make voltage-driven-type devices having a MOS structure to be used practically, it is necessary to reduce the interface state density at the interface between a silicon carbide substrate and a silicon oxide film and the flat band voltage Vfb of the silicon carbide substrate.
In detail, for obtaining interface properties excellent enough to make voltage-driven-type devices having a MOS structure to be used practically, it is necessary to reduce the interface state density at the interface between a silicon carbide substrate and a silicon oxide film down to the order of 1×1011 cm−2·eV−1 and to bring the flat band voltage Vfb of the silicon carbide substrate close to zero. The flat band voltage Vfb is the potential variation of the silicon carbide substrate changing from the flat band state caused by bonding the silicon carbide substrate and the silicon oxide film to each other.
By the technique reported by N. S. Saks, et al 17 Apr. 2000, the channel mobility becomes low, since the interface state density is high, around 1×1013 cm−2·eV−1, at the interface between a silicon carbide substrate and a silicon oxide film near the conduction band of the silicon carbide substrate as shown in FIG. 3 of N. S. Saks, et al 17 Apr. 2000. By the technique reported by Y. Iwasaki, et al 2010, the threshold voltage Vth becomes high, since the flat band voltage Vfb of the silicon carbide substrate is around 6.4 V as listed in Table 1 of Y. Iwasaki, et al 2010.
The technique reported by M. Noborio, et al AUGUST 2008 makes the interface state density at the interface between the silicon carbide substrate and the silicon oxide film and the flat band voltage Vfb of the silicon carbide substrate meet the conditions described above for obtaining interface properties excellent enough to make the voltage-driven-device to be used practically. However, it is necessary for the technique to oxidize the silicon nitride film deposited on the silicon carbide substrate by the CVD method.
As described in FIG. 1 of M. Noborio, et al AUGUST 2008, it is necessary to conduct an anneal treatment in a dinitrogen monoxide (N2O) gas atmosphere at 1300° C. for oxidizing the silicon nitride film in addition to the anneal treatment in a nitrogen (N2) gas atmosphere at 1300° C. Therefore, it is necessary for the technique to conduct anneal treatments twice at a high temperature. It is also necessary for the technique described by N. S. Saks, et al 17 Apr. 2000 to conduct anneal treatments twice at a high temperature.
In view of the foregoing, it would be desirable to obviate the problems described above. It would be also desirable to provide a method of manufacturing a semiconductor device that exhibits excellent interface properties including an interface state density and a flat band voltage.